In commonly used electric storage batteries, such as the well known 12-volt battery employed in cars, it has been a desiratum to have a battery separator between the battery plates as thin as is possible to obtain so as to have the lowest possible electrical resistance. At the same time, it has been sought to obtain a battery separator which is reasonably flexible and yet does not develop failure in use such as brittle failure.
Generally, a battery separator is needed as a spacer to prevent two plates from touching each other causing a short. At the same time, a separator shall not impede the electrolyte flow. Also, a fine pore size is desirable to prevent dendrite growth developing between adjacent plates. The result of dendrite growth is a battery "short". For one or more of the reasons given above, it has been necessary not only to increase the battery plate spacing, but also to use battery separators.
Various other problems have also resulted from spalling of the battery plates associated with the use of antimony or calcium additives to the lead plates. Spalled deposits at the bottom of the battery have likewise caused shorts or premature failure of the battery. For this reason, it has been sought to have a battery which could be made in a manner whereby the battery separators could be festooned around the plates or made in a serpentine fashion thereby isolating one plate from the other.
However, the prior art battery separators have been invariably rather stiff and inflexible; complex shapes could only be formed with great difficulty. In addition to the above problems, overvoltage caused at the electrodes, particularly at an anode, has required the well known addition of battery water.
Only recently the overvoltage problem has been solved to a point such that maintenance-free batteries can be used with any degree of satisfaction. In no small part this has been a result of better plate or electrode materials or battery separators.
In the art of producing battery separators, commonly as a cheap and fairly short-lived separator, paper webs have been used. However, these possess disadvantages and instead of paper, better quality batteries have, as separators, cured natural rubber compositions. A common disadvantage inherent in the use of rubber or natural rubber based battery separators is that a sulfur cure process not only is capital intensive, being a batch process, but it is also labor and energy intensive. Sulfur curing of natural rubber microporous articles results in stiff and brittle products. Moreover, in order to maintain the porosity provided by rehydrated silica, a battery separator must be sulfur cured in a water filled autoclave. Repeated raising and lowering of temperature of large amounts of water is very energy consuming.
In the curing of the rubber compositions, the cured articles are tested for cracking and brittleness. Unless very careful processing steps are followed in making sulfur cured separators, problems of brittle cracking often result. Dimensional tolerances are also difficult to maintain, for example, cured sheets from which battery separators are made require grinding.